import java.util.Scanner;

public class Dva{
     public static void main(String []args){
        Scanner sc = new Scanner(System.in);
        int vsota = 0;
        int ena = sc.nextInt();
        for(int i = 0; i <= ena; i++){
            if(i % 5 == 0){
                vsota += i;
            }
        }    
        System.out.println(vsota);
    }
}